There are two methods for separating cells from cell-containing liquids such as blood: the centrifugation method which uses the difference in specific gravity of cells; and the filtration method in which a porous body is used as a filter material. The filtration method which offers high cell separation properties by a simple operation at low cost has been widely and commonly used. In the current market, there are needs for filters having higher cell separation properties. However, filters according to conventional techniques cannot easily achieve cell separation properties that are high enough to meet the market needs by simply increasing the amount of filter material. To solve this problem, various improvements have been reported in filters using nonwoven fabrics as filter materials. Examples include a method of decreasing average fiber diameter, a method of increasing packing density, a method of uniformizing fiber diameter distribution or pore size, a method of reducing variations in pore size distribution in a mixture of fibers of different thicknesses, and a method of uniformizing both fiber diameter and basis weight (Patent Literatures 1 to 4). However, if the above methods are actually tried to increase cell separation properties, the increase in cell separation properties will be accompanied by an increase in pressure loss in the filter portion during passage of cell-containing liquids, which unfortunately results in an extreme reduction in throughput rate before the completion of the filtration of a desired amount of cell-containing liquid to be treated.
Moreover, in addition to uniformization of a single property element such as, typically, fiber diameter or average pore size, studied have been made on the standard deviation of fiber density of the entire filter material, and a method of reducing the fiber density variation difference of the entire filter material, or in other words a filter material having a low formation index has been reported (Patent Literature 5). However, even filter materials having low formation indices may fail to provide sufficient cell separation properties due to flowing of the cell-containing liquid on one side. In addition, the concentrated flow of cell-containing liquid on a portion of the filter causes clogging, which unfortunately results in a longer treatment time.
As described above, although many filters for separating cells have been examined, the problems such as the reduction in throughput rate associated with the improvement of cell separation properties and clogging in filters have not yet been solved.